DIY: Fiberglass Mold

by Chris Gilliland

From motorcycles to muscle cars, fiberglass can be used in many ways to produce custom parts and body panels at a relatively low cost. For years many enthusiasts have turned to fiberglass to take advantage of this inexpensive composite material's strength and accessibility to create one-off pieces. Mass production of these parts, however, requires the construction of a reusable mold to ensure quality in every piece. Creating a mold takes careful planning as there are many factors to consider.

Mold Types

The first step is to decide on the actual form of the mold needed. You may opt to create either a male or female type mold. The best solution depends on your needs. Male molds are suitable for quick, small production parts or non-cosmetic parts, such as braces or brackets. If you plan on producing large amounts of cosmetic pieces, female molds may be worth the investment of time and money.

Male (Positive) Mold

A male, or positive mold offers the quickest way to form your part by layering fiberglass directly onto the exterior of the mold. However, a positively molded piece requires a fair amount of surface finishing. Another consideration is that positive molding can also enlarge the part as layers of fiberglass build up over the mold. To prevent this, some mold makers may build their mold smaller to accommodate the increase in size.

Female (Cavity) Mold

A female, or cavity mold allows the user to layer fiberglass along the interior of the mold. A properly constructed female mold will impart a smooth, even surface on the molded part, requiring less finishing after removal. The main drawback to female molds lies in the construction process, requiring the mold to be cast from a male mold first. Alternatively, precise construction of parts can be made by using both male and female components to create a compression-type mold. In this application, fiberglass and resin are compressed between both molds, forcing out excess resin and air to create a smooth surface on both sides of the piece.

Mold Construction

The first step in creating your mold is to create a plug, or the basic form of your part. This can be done either by using an existing part or by fabricating the plug from easily shaped materials, such as clay, cardboard, polystyrene foam or even spray foam. Build your plug according to the type of mold chosen. Remember that a male mold will produce a larger part, so build your plug smaller than the actual size of the piece. Once shaped it is best to mount the plug to a flat board and cover with a layer of fiberglass and resin. The board will make the new piece easier to remove from the finished mold. Once the plug has cured, sand it down to create as smooth a surface as possible and fill in any low spots with body filler. This will save you a lot of time on the final finishing. Cover your plug with a layer of tooling gel, which will allow you to fine-tune the mold's surface once removed from the plug, and allow it to cure. Finally build up the mold using fiberglass and resin, covering the entire plug and board.

Using the Mold

Begin by cleaning out the mold using soap and water, then apply a layer of mold release wax. Spray a thin layer of gel coat and allow to dry for a few minutes. Build up your piece with fiberglass and resin, again extending the fabric out onto the edges of the mold to aid the removal process and let it cure. Once removed, wash off any excess release wax with soap and water. Your new part is ready to prep and paint.

About the Author

An avid motorcyclist, Chris Gilliland has immersed himself into the two-wheeled world while balancing work life and raising three daughters. When he is not managing the parts department of a local, multi-line motorcycle dealership, Gilliland can often be found riding, writing or working on his motorcycle blog, Wingman's Garage.